1. Field of the Invention
This invention relates to vascular prostheses of polytetrafluoroethylene and a water-insolubilized water-soluble polymer.
2. Description of the Prior Art
Vascular prostheses made of a knitted or woven fabric of a polyester (e.g., Dacron, trade name produced by E. I. du Pont de Nemours & Co. Inc.) or polytetrafluoroethylene are currently utilized and those having a relatively large inside diameter are practical and have been used with a high degree of success. In the case of arterial vascular prostheses, the results are excellent if their inside diameter is greater than about 7 mm. However, few vascular prostheses of fine diameters are clinically acceptable. Particularly, in venous applications, the degree of success is lower than in arterial applications. The blood flow in veins is slower than in arteries, and in veins, the inhibition of platelet adhesion is especially important to prevent thrombosis. Vascular prostheses in current use do not fully meet this requirement.
It is known that some polytetrafluoroethylene tubes produced by stretching or expansion can be clinically used as vascular prostheses in arteries and veins [e.g., as disclosed in Soyer et al., "A New Venous Prosthesis," Surgery, Vol. 72, p. 864 (1972); Volder et al., "A-V Shunts Created in New Ways," Trans. Amer. Soc. Artif. Int. Organs, Vol. 19, p. 38 (1973); Matsumoto et al., "A New Vascular Prosthesis for a Small Caliber Artery," Surgery, Vol. 74, p. 519, (1973), and "Application of Expanded Polytetrafluoroethylene to Artificial Vessels," Artificial Organs, Vol. 1, p. 44 (1972), ibid., Vol. 2, p. 262 (1973) and ibid., Vol. 3, p. 337 (1974); Fujiwara et al., "Use of Goretex Grafts for Replacement of the Superior and Inferior Venae Cavae," The Journal of Thoracic and Cardiovascular Surgery, Vol. 67, p. 774, (1974); and Belgian Pat. No. 517,415].
The results of these clinical tests are summarized below.
When a suitable prosthesis is implanted as a conduit within the arterial system, the fine pores of the vessel are clogged by clotted blood, and the inside surface of the vessel is covered by a layer of the clotted blood. The clotted blood layer is made of fibrin, and the thickness of the layer varies according, for example, to the material and surface structure of the blood vessel. When a knitted or woven fabric or a polyester such as Dacron or polytetrafluoroethylene is used, the fibrin thickness approaches about 0.5 to about 1 mm. Accordingly, such a prosthesis is successful only with blood vessels having a caliber such that occlusion due to a thickening of the fibrin layer does not occur, namely with arteries having a inside diameter of 5 to 6 mm or more. Generally, vascular prostheses made of woven or knitted fabrics are not successful when the inside diameter is small.
On the other hand, polytetrafluoroethylene tubes which have been stretched have a micro-structure of very fine fibers and nodes connected to one another by the fibers. The diameter of the fibers, which varies according to the stretching conditions, can be made far smaller than the diameters of the fibers for the woven or knitted fabrics described above.
This structure of fibers and nodes can be described in terms of pore size, porosity, fiber length and nodular size. It has been clinically confirmed that with polytetrafluoroethylene tubes defined by a pore size of about 2.mu. to about 30.mu. (pore sizes of less than about 2.mu. are not preferred), a porosity of about 78% to about 92%, a fiber length of not more than about 34.mu. (fiber lengths of about 40 to about 110.mu. are not preferred), a nodular size of not more than 20.mu., and a wall thickness of about 0.3 mm to about 1 mm, little occlusion by fibrin deposition occurs, and a high patency rate is exhibited.
It was reported, however that the patency rate in venous prostheses is far lower than that in arterial prostheses. Thus, a complete vascular prosthesis for veins has not been obtained. It was also reported that when the porosity of such a prosthesis is too high, the suture used in joining the prothesis to the vessel in a patient tends to tear the prosthesis.
In the healing process after implantation in the living body, the periphery of the polytetrafluoroethylene tube first organizes by being enveloped in connective tissue, and the inside fibrin layer organizes after the periphery. At this time, the intimas at both ends of the host's vessel extend to the inside surface of the vascular prosthesis, and the fibrin layer is replaced by the fibrous tissue which has come from the periphery of the prosthesis through the fine pores. After a certain period of time, the neo-intima in the inside surface is firmly connected to the connective tissue at the periphery thereby to complete the formation of an artery. It is known that this period is generally 4 to 6 months. It is also known that in the case of a vascular prosthesis implanted in a vein, the speed of entry of the connective tissue from the periphery is slower than in the case of arteries.
The expected mechanism of a feasible vascular prosthesis of a polytetrafluoroethylene tube is that the porous polytetrafluoroethylene tube adsorbs plasma protein, platelets adhere to the protein to form fibrin fibers which capture blood corpuscles and become a fibrin-deposited layer, and then the deposited layer forms a pseudo-intima of the vascular prosthesis. However, the thickness of the fibrin-deposited layer frequently becomes too large, and nutrient supply to the pseudo-intima or neo-intima becomes insufficient. This results in a calcification of the prosthesis wall or the occlusion of the inner cavity of the prosthesis.